GENEVA (Reuters) -- United Nations Secretary-General Ban Ki-moon called on October 5 for world leaders to display political will to work towards genuine reductions and an eventual ban of nuclear weapons.

Addressing the same forum at the UN's European headquarters, former president of the old Soviet Union Mikhail Gorbachev echoed the plea, declaring: "Nuclear weapons must be destroyed. We must rid ourselves of this threat."

"The greatest challenge in nuclear disarmament has always been the task of bridging the gap between words and deeds," declared Ban, speaking in the wake of a UN Security Council call last month calling for scrapping nuclear arsenals.

That call was issued in a resolution proposed by the United States as President Barack Obama chaired a special summit session of the 15-member Council organised by Ban to get long-stalled disarmament discussions moving.

Ban said there were five central challenges confronting the international community on the road to scrapping nuclear weapons, four of them substantive issues of disarmament and the shape accords should take.

"But there is a fifth and perhaps even greater challenge that precedes all the others: the challenge of exercising much needed political will," he told an invited audience of diplomats, academics and members of the public.

The September 24 Security Council resolution called for efforts to achieve "a world without nuclear weapons." But critics noted that it included no mandatory provisions requiring concrete steps by nuclear weapons states.

Ban said any commitment to eliminate nuclear weapons "can only be achieved with the most binding of all legal obligations," perhaps in the form of a multilateral convention. "Whatever the form, it must have the force of law," he added.

The UN chief said Gorbachev, main speaker at the meeting and Kremlin chief as the Soviet Union and the United States reached key arms treaties in 1985-91, had been a "giant" showing leadership in the drive for a nuclear-free world.

Gorbachev, who lost his post when the Soviet state collapsed at the end of 1991 and has little influence in Russia today, said the major nuclear powers -- the United States, Russia, China, France and Britain -- should declare a freeze on their weapon stocks to get talks going on their elimination.

He rejected a suggestion from the audience that nuclear weapons could help maintain security. "It is a great delusion that they helped to preserve peace in the past," he said. There was always the threat of accidental nuclear war.